Cyclotriphosphazene-BODIPY Dyads: Synthesis, halogen atom effect on the photophysical and singlet oxygen generation properties

Journal Pre-proofs Research paper Cyclotriphosphazene-BODIPY Dyads: Synthesis, halogen atom effect on the photophysical and singlet oxygen generation properties Elif Şenkuytu, Elif Okutan, Esra Tanrıverdi Eçik PII: DOI: Reference:

S0020-1693(19)31569-5 https://doi.org/10.1016/j.ica.2019.119342 ICA 119342

To appear in:

Inorganica Chimica Acta

Received Date: Revised Date: Accepted Date:

15 October 2019 2 December 2019 2 December 2019

Please cite this article as: E. Şenkuytu, E. Okutan, E. Tanrıverdi Eçik, Cyclotriphosphazene-BODIPY Dyads: Synthesis, halogen atom effect on the photophysical and singlet oxygen generation properties, Inorganica Chimica Acta (2019), doi: https://doi.org/10.1016/j.ica.2019.119342

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© 2019 Published by Elsevier B.V.

Cyclotriphosphazene-BODIPY Dyads: Synthesis, halogen atom effect on the photophysical and singlet oxygen generation properties

Elif Şenkuytua, Elif Okutana, Esra Tanrıverdi Eçikb,*

aDepartment

of Chemistry, Gebze Technical University, Gebze, Kocaeli, Turkey

bDepartment

of Chemistry, Atatürk University, Erzurum, Turkey

E-mail: [email protected]; Tel: +90 442 231 44 96

KEYWORDS: Cyclotriphosphazene, BODIPY, Photosensitizer, Singlet Oxygen, X-ray

1

ABSTRACT

Two new dendrimeric cyclotriphosphazenes (CBD 1 and CBD 2) bearing six BODIPY units functionalized with halogens were successfully designed and synthesized. Their photophysical properties including absorption and emission profiles, fluorescence quantum yield, and fluorescence lifetime were investigated. The dendrimeric cyclotriphosphazene-BODIPY systems displayed intense absorption bands at about 530 nm with good molar extinction coefficients and weak emissions owing to the presence of halogen atoms in the structures. We also determined the singlet oxygen (1O2) formation abilities of the dendrimeric systems by chemical trapping and NIR phosphorescence methods. It was observed that the iodinatedBODIPY-cyclotriphosphazene dyad (CBD 1) was more efficient at generating 1O2 than that of the brominated-dyad (CBD 2). Besides, both dendrimeric systems showed more remarkable photosensitization ability than some commonly used photosensitizer based on porphyrin and BODIPY derivatives.

2

1.

Introduction

Triplet photosensitizers (TPS) have attracted the attention of the researchers because of their potential applications in photodynamic therapy (PDT), upconversion systems, photocatalysis and photovoltaics [1-4]. A suitable TPS should have high molar absorption coefficients of the excitation light, high photostability and efficient intersystem crossing (ISC) to produce long triplet excited state [2, 5]. The majority of photosensitizers (PS) has focused on enhancing intersystem crossing efficiency and production of singlet oxygen [2, 4, 5]. Boron dipyrromethene (BODIPY) derivatives have been one of the fundamental molecule groups as photosensitizer in recent years due to their excellent photophysical properties, photostability and flexible structure [2, 4, 6-9]. Covalent attachment or coordination of a heavy atom, such as a transition metal or halogen, to the BODIPY unit have been frequently used to improve the ISC for triplet photosensitization [2-4]. Nowadays, many dendrimeric systems, with unique core, branches, internal cavities and terminal groups, have been reported as potential photosensitizer candidate [10-13]. Most of the dendrimer cores maintain di-, tri and tetra-branches, linked to the center [11]. Cyclotriphosphazenes (trimer), a remarkable family of inorganic hetero-systems, have robust phosphorus–nitrogen backbone and the ring carry six active P-Cl bonds, facilitating readily available six branches [14,15]. As a thermally and chemically stable scaffold trimer can also be easily functionalized via substitution reactions of the chloride atoms with functional groups. These features make the cyclotriphosphazene ring an ideal core to design dendrimeric molecules [14-18]. Recent interest in dendrimeric cyclotriphosphazenes carrying chromophore group have increased considerably to develop new materials for special application areas [16, 17, 19-21]. Especially, cyclotriphosphazenes functionalized with BODIPY, perylene and fullerene units exhibit remarkable photophysical and photochemical properties one of which is the ability of efficient production of singlet oxygen [16, 22, 23].

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In this study, new dendrimeric cyclotriphosphazenes bearing brominated or iodinated BODIPY units were synthesized and characterized. The photophysical properties of the dendrimeric systems were investigated. The influence of halogen atom (I and Br) substitution in chromophore unit on singlet oxygen production efficiencies of the dendrimeric systems was also elucidated. 2. Experimental section 2.1. General methods All reagents and solvents were purchased from commercial suppliers and used without further purification. Analytical thin layer chromatography (TLC) was performed on silica gel plates (Merck, Kieselgel 60 Å, 0.25 mm thickness) with F254 indicator. Column chromatography was performed on silica gel (Merck, Kieselgel 60 Å, 230-400 mesh). Mass spectra were acquired in linear modes with average of 50 shots on a Bruker Daltonics Microflex mass spectrometer equipped with a nitrogen UV-Laser operating at 337 nm. 1H, 13C and 31P NMR spectra were recorded for new compounds in CDCl3 solutions by a Varian INOVA 500 MHz spectrometer. Electronic absorption spectra in the UV-Vis region were recorded with a Shimadzu 2101 UVVis spectrophotometer. Fluorescence excitation and emission spectra were recorded by a Varian Eclipse spectrofluorometer using 1.0 cm path length cuvettes at room temperature. Fluorescence lifetimes were measured using a time correlated single photon counting setup (TCSPC; Horiba Fluorolog 3 equipment) HORIBA Model: FLUOROLOG 3. Signal acquisition was performed using a TCSPC module (NanoLED -390 nm).

4

2.2. The parameters for fluorescence quantum yields The fluorescence quantum yield (ΦF) values of CBD1 and CBD 2 were determined in dichloromethane by comparing to the fluorescence of Rhodamine 6G (ΦF = 0.95 in water) as a reference molecule (Eq. 1) [24, 25].

ΦF  ΦF(Std)

F . AStd . n 2 2 FStd . A . n Std

(1)

where ΦF(Std) is the fluorescence quantum yield of the reference molecule. F and FStd are the areas under the fluorescence emission curve of dyads (CBD 1 and CBD 2) and the reference, respectively. A and AStd are the respective absorbance of the compounds and the reference at 2 the excitation wavelengths. η 2 and ηStd are the refractive indices of solvents used for the sample

and the compounds. The concentration of the solutions at the excitation wavelength fixed to 2x10-6 mol.dm-3.

2.3. Singlet Oxygen Measurements Singlet oxygen quantum yield (φΔ) was determined by the comparative method described previously [26]. Methylene Blue (MB) was used as a reference molecule and 1,3diphenylisobenzofuran (DPBF) was used as a singlet oxygen catcher. By following the absorption of DPBF, the singlet oxygen production was monitored. The following equation was used to calculate the singlet oxygen quantum yields of the samples. φΔ (samp) = φΔ (ref) × [m (samp)/m(ref)]×[F(ref)/F(samp)] where m is the slope of difference in change in absorbance of DPBF (414 nm) with the irradiation time and F is the absorption correction factor, which is given by F = 1 – 10-OD (OD at the irradiation wavelength).

5

Singlet oxygen formation (ΦΔ) measurements via the phosphorescence of 1O2 were measured by using Horiba Jobin-Yvon Fluorimeter with Hamamatsu NIR PMT 5509. The ability of singlet oxygen formation was obtained via according to the equation,

𝑰∆ (𝒅𝒚𝒂𝒅) 𝑨(𝒔𝒕𝒅)

(2)

Ф∆ = Ф∆(𝒔𝒕𝒅)𝒙 𝑰∆ (𝒔𝒕𝒅) 𝑨(𝒅𝒚𝒂𝒅)

where, ΦΔ (std) is the singlet oxygen quantum yield of the standard (MB, 0.57 in DCM) [27]; IΔ is the area under the curve of signature peak of singlet oxygen at 1270 nm; and A is the absorbance which was set to 0.236 for CBD1, CBD2 and MB.

2.4. Synthesis Compound 1, 2 and 4 were synthesized according to literature [16].

2.4.1. Synthesis of Compound 3 Compound 1 (280.0 mg, 0.59 mmol) was dissolved in CH2Cl2:DMF (10:2 mL). Solution of Nboromosuccinimide (NBS; 316.1 mg, 1.78 mmol) in CH2Cl2 (10.0 mL) was added dropwise and the reaction mixture was stirred for 2 h at room temperature. After the extraction with water, organic layer was dried with Na2SO4 and evaporated under vacuo. The crude product was purified by silica gel column chromatography using CH2Cl2:n-hexane- (2:1) as mobile phase. The fraction containing compound 3 was collected then the solvent was removed under reduced pressure (0.42 mmol, 250.0 mg, 71.0 %). MS (MALDI-TOF) m/z Calc. 595.05; found 595.32 [M+H]+ (Fig. S1). 1H NMR (500 MHz, CDCl3) δH 7.15 (d, J = 8.4 Hz, 2H), 7.04 (d, J = 8.4 Hz, 2H), 4.08 (t, J = 6.0 Hz, 2H), 3.42 (t, J = 6.6 Hz, 2H), 2.62 (s, 6H), 2.00-1.91 (m, 2H), 1.90-

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1.82 (m, 2H), 1.41 (s, 6H) ppm (Fig. S2). 13C NMR (126 MHz, CDCl3) δC 159.8, 153.7, 142.2, 140.6, 130.8, 129.1, 126.3, 115.2, 111.6, 67.3, 51.2, 26.5, 25.7, 13.9, 13.6 ppm (Fig. S3).

2.4.2. Synthesis of CBD 1 Compound 2 (80.0 mg, 0.12 mmol) and compound 4 (8.99 mg, 0.02 mmol) were dissolved in CH2Cl2:CH3OH:H2O (4.0 mL:1.0 mL:1.0 mL) mixture. Sodium ascorbate (7.5 mg; 0.03 mmol), CuSO4.5H2O (6.0 mg, 0.03 mmol) and one drop of triethylamine were added and the reaction mixture was stirred at room temperature for 2 days. After the reaction was completed, extraction was performed by CH2Cl2 and water. Organic layer was collected, dried with sodium sulfate, and the solvent was evaporated. The crude product was purified by silica gel column chromatography using CH3OH-CH2Cl2 (5:100) as mobile phase. The fraction containing compound CBD 1 was collected; then, the solvent was removed under reduced pressure (0.006 mmol, 27.0 mg, 31%). 31P NMR (proton decoupled; 202 MHz, CDCl3) δP 15.10 (s, 3P), ppm (Fig. S4). 1H NMR (phosphorus coupled; 500 MHz, CDCl3) δH 7.60 (s, 6H), 7.14 (d, J = 8.1 Hz, 12H), 7.01 (d, J = 8.1 Hz, 12H), 4.61 (s, 12H), 4.50 (t, J = 6.9 Hz, 12H), 4.06 (t, J = 5.8 Hz, 12H), 2.65 (s, 36H), 2.24-2.15 (m, 12H), 1.95-1.84 (m, 12H), 1.41 (s, 36H) ppm (Fig. S5). 13C

NMR (126 MHz, CDCl3) δC 159.7, 156.5, 145.3, 142.3, 141.4, 131.7, 129.1, 126.8, 122.1,

115.2, 110.2, 67.1, 66.1 49.9, 27.2, 26.2, 17.2, 16.1 ppm (Fig. S6).

2.4.3. Synthesis of CBD 2 Compound 3 (100.0 mg, 0.17 mmol) and compound 4 (13.0 mg, 0.03 mmol) were dissolved in CH2Cl2:CH3OH:H2O (4.0 mL:1.0 mL:1.0 mL) mixture. Sodium ascorbate (7.5 mg, 0.03 mmol), CuSO4.5H2O (6.0 mg, 0.03 mmol), and one drop of triethylamine were added and the reaction mixture was stirred at room temperature for 2 days. After the reaction was completed, extraction was carried out by CH2Cl2 and water. Organic layer was collected, dried with sodium sulfate,

7

and the solvent was evaporated. The crude product was purified by silica gel column chromatography using CH3OH-CH2Cl2 (5:100) as mobile phase. The fraction containing CBD 2 was collected; then, the solvent was removed under reduced pressure (0.01 mmol, 38.0 mg, 34%). 31P NMR (proton decoupled; 202 MHz, CDCl3) δP 14.99 (s, 3P), ppm (Fig. S7). 1H NMR (phosphorus coupled; 500 MHz, CDCl3) δH 7.60 (s, 6H), 7.15 (d, J = 7.5 Hz, 12H), 7.02 (d, J = 7.5 Hz, 12H), 4.61 (s, 12H), 4.50 (t, J = 6.6 Hz, 12H), 4.07 (t, J = 6.2 Hz, 12H), 2.65 (s, 36H), 2.23-2.15 (m, 12H), 1.93-1.83 (m, 12H), 1.41 (s, 36H) ppm (Fig. S8). 13C NMR (126 MHz, CDCl3) δC 159.7, 153.7, 145.30, 142.1, 140.5, 130.7, 129.1, 126.5, 122.1, 115.2, 111.6, 67.1, 66.1 49.9, 27.2, 26.2, 13.9, 13.6 ppm (Fig. S9).

8

N3

O

N

N l no

O

3

ha

HI

(1)

I2 ,

Et

l2

NB

,D

M

F

S

N3

I

N

N

2C

F

F

N3

O

I

CH

B

O

Br

B F

F

+

(2)

B

O

O P

N

P

N

N

+

O O

O ICK

CL

CL

(4)

F F B N N

F F N B N

R

R

R

R

N N N

F

N N

N

P N

N

N N N

O

N

O

O

R

B

R

O

O

F

F F (3)

P

ICK

O

Br

N

N

O

O P

P N O N

N

R

N

F N B F N

N

O

O N N N O

O

R N B N F F

R R

R

N N N

R N N B F F

(CBD 1) R = I (CBD 2) R = Br

Scheme 1. Synthesis pathway of CBD 1 and CBD 2.

9

3. Results and Discussion 3.1. Synthesis and structural characterization of CBD 1 and CBD 2 The synthesis strategies towards CBD 1 and CBD 2 were outlined in Scheme 1. First, BODIPY derivative (1) functionalized with azido (-N3) group on meso position was prepared via classic one pot procedure including reaction of readily available aldehyde derivative with 2,4dimethylpyrrole followed by the oxidation using p-chloranil to form dipyrrin, which in turn complexed with BF3.Et2O in the presence of Et3N. To facilitate the intersystem crossing, 2 and 6 positions of BODIPY core (1) were iodinated by using iodic acid and iodine in ethanol at 50C and brominated in the presence of NBS in CH2Cl2 at room temperature to synthesize BODIPY derivatives 2 and 3. The next step involved the preparation of cyclotriphosphazene core

(4)

for

click

reaction

via

nucleophilic

substitution

reaction

of

hexachlorocyclotriphosphazene with propargyl alcohol in THF. Finally, the treatment of compound 4 with di-halogenated BODIPYs (2 and 3) in the presence of sodium ascorbate and copper (II) sulfate in dichloromethane-methanol mixture at room temperature resulted in CBD 1 and CBD 2, respectively. All new compounds (3, CBD 1 and CBD 2) were characterized by 1H, 13C

and 31P NMR. The molecular structures of compounds 2 and 3 were also determined

by X-ray crystallography. The 1H NMR spectra of di-brominated BODIPY (3) and cyclotriphosphazene-BODIPY dyads (CBD 1 and CBD 2) exhibited doublet signals for meso aromatic protons at around 7.2-7.0 ppm region (Fig. S2, S5 and S8). The coupling constants (3JHH) of compound 3, CBD 1 and CBD 2 were calculated as 8.4, 8.1 and 7.5 Hz, respectively. The triazole ring -NCH protons for CBD 1 and CBD 2 appeared as sharp singlets at 7.6 ppm. The aliphatic –OCH2 protons of the compounds (3, CBD 1 and CBD 2) showed triplet peaks at approximately 4.1 ppm with similar coupling constants (3JHH = 6.0 Hz, 3JHH = 5.8 Hz and 3JHH = 6.2 Hz, respectively). The POCH2 protons of the CBD 1 and CBD 2 gave singlet peaks at 4.6 ppm. The - CH2N3 protons for 3

10

were observed at 3.4 ppm (3JHH = 6.6 Hz) whereas the -NCH2- protons for dyads (CBD 1 and CBD2) were seen as triplets at 4.5 ppm. The coupling constants of CBD 1 and CBD 2 were calculated as 6.9 and 6.6 Hz. The -CH3 protons on the BODIPY cores were observed at 2.6 and 1.5 ppm as sharp singlet peaks and the aliphatic -CH2 protons were obtained between 2.2-1.8 ppm as multiplets for all compounds (3, CBD 1 and CBD 2), and the integrals values were also confirmed the proposed structures (Fig. S2, S5 and S8). The proton-decoupled 31P NMR spectra of cyclotriphosphazene-BODIPY dyads (CBD 1 and CBD 2) were given in Fig. S4 and S7. CBD 1 and CBD 2 showed A3 spin systems as expected and the resonance belong to the three phosphorus atoms were observed as only one signal at about 15.0 ppm. In the 13C NMR spectra of the newly synthesized compounds, the aromatic carbons were observed between 159.8-110.2 ppm where the aliphatic carbons appeared between 67.1-13.6 ppm region (Fig. S3, S6 and S9). The solid-state structures and geometries of the compounds 2 and 3 were also determined by using single crystal X-ray structural analysis. Although the compound 2 was synthesized according to the literature [16], the structure of di-iodinated BODIPY (2) was characterized for the first time with single crystal X-ray in this study. The crystallographic data and refinement parameters were summarized in Table S1. ORTEP representations of these two structures were represented in Fig. 1 and 2. The compounds 2 and 3 crystallized in the monoclinic P21/c and triclinic P-1 space groups, respectively. In the crystal structures of 2 and 3, there were four and two molecules at unit cell, respectively (Fig. S10 and S11). The bond distances B1-F1 and B1F2 were 1.377(11) Å and 1.358(10) Å for compound 2; and 1.379(6) Å and 1.374(6) Å for compound 3. The bond angles F2-B1-F1 and N4-B1-N5 were found to be 110.9(7) ° and 107.0(4)° for compound 2; and 109.1(4)° and 105.9(7)° for compound 3. In addition, the NB-F av. bond angles of compounds (2 and 3) were found as 109.875°. Some bond lengths, bond

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angles and conformational parameters were found to be similar to those observed for the previously reported BODIPY derivatives [28-30]. The selected bond lengths and angles are given in Table S2 and S3.

Figure 1. ORTEP drawings of the compound 2 (40% probability level). The grey, red, blue, pink, green and purple colored atoms represent C, O, N, B, F and I, respectively.

Figure 2. ORTEP drawings of the compound 3 (40% probability level). The grey, red, blue, pink, green and brown colored atoms represent C, O, N, B, F and Br, respectively.

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3.3. Photophysical properties of CBD 1 and CBD 2 The ultraviolet visible (UV−vis) spectra of cyclotriphosphazene-BODIPY dyads (CBD 1 and CBD 2) in dichloromethane at room temperature were shown in Figure 3. The absorption band of CBD 1 was centered at 535 nm (=6.37×104 M-1 cm-1) while the absorption band of CBD 2 was at 529 nm (=5.26×104 M-1 cm-1), which can be assigned to the S0-S1 transitions. The molar absorption coefficient () of CBD 1 was seen to be higher than that of CBD 2 (Fig. S12 and S13). Stokes shifts of the dendrimeric systems were about 15 nm. CBD 2 showed an emission maximum at 550 nm when excited at 530 nm with florescence quantum yield of 0.072 (Table 1). CBD 1 exhibited emission maximum at 544 nm (ex =525 nm) with a very low quantum yield value of 0.042. Although the fluorescence quantum yield decreased depending on the size of the halogen atom, residual fluorescence could still be detected (Table 1). Fluorescence lifetimes (τF) of the compounds were directly measured with monoexponential calculation (Fig. S14). The lifetime values were determined to be 0.19 ns and 1.73 ns for CBD 1 and CBD 2, respectively. The decrease in the fluorescence lifetime and quantum yield of dendrimeric cyclotriphosphazenes bearing iodinated BODIPY unit (CBD 1) compared to CBD 2 can be attributed to more potent heavy atom atom effect as the halogen atoms support intersystem crossing (ISC) and quench the excited state [2].

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500

Exct. CBD 1 Ems. CBD 1 Exct. CBD 2 Ems. CBD 2

Intnesity

400 300 200 100 0 425

475

525 575 Wavelength (nm)

625

675

Figure 3. Excitation and emission spectra of CBD 1 (λex= 525 nm) and CBD 2 (λex= 530 nm) (0.5 µM) in dichloromethane. Table 1. Photophysical and photochemical properties of CBD 1 and CBD 2a Compound

λab, nm

λem, nm

єb , 104 M-1 cm-1

∆Stokes, (nm)

CBD 1

535

550

6.37

15

CBD 2

529

544

5.26

15

τF (ns)c 0.19 (CHISQ=0.65)

1.73 (CHISQ=0.83)

ΦFd

φ∆e

0.042

0.83

0.072

0.64

Dichloromethane. b Molar extinction coefficients. c Lifetime, d Fluorescence quantum yield, e Singlet oxygen quantum yield. a

3.2. Photochemical properties of CBD 1 and CBD 2 The singlet oxygen (1O2) generation capacities of the dendrimeric cyclotriphosphazenes (CBD 1 and CBD 2) were determined by two different techniques. Firstly, we compared the 1O2 production abilities of CBD 1 and CBD 2 using a common chemical trapping method in which 1,3-diphenylisobenzofuran (DPBF) was preferred as the trapping agent for 1O2 and methylene blue as a standard sensitizer. Initially, each of the mixtures including compounds (CBD 1 and CBD 2) and DPBF was kept at the dark for 20 min to eliminate contribution to the absorbance changes from dark reactions. As expected, the absorption band of 1,3-diphenylisobenzofuran

14

at 414 nm for both dendrimeric compounds did not exhibit any deviation at dark. When the light was turned on, dendrimeric cyclotriphosphazenes started to produce singlet oxygen and the characteristic absorption band of DPBF decreased systematically (Fig. 4 and S15). The solution contain dendrimeric cyclotriphosphazenes (CBD 1 or CBD 2; 0.5 M) and DPBF (35.0 M) was irradiated with green LED (= 516 nm, 2.1 mW cm-2) for a time interval between 0 to 35 sec. The data for CBD 1 or CBD 2 were plotted as the change in absorption of DPBF versus irradiation time (Fig. 4 and S15). Based on the DPBF oxidation data, singlet oxygen quantum yields (φΔ) were calculated as 0.83 and 0.64 for dendrimeric cyclotriphosphazenes bearing iodinated (CBD 1) and brominated (CBD 2) BODIPY units respectively. Halogen substitution is the most common used method to prepare new singlet oxygen photosensitizers [2, 4, 31, 32]. It was reported that singlet oxygen quantum yield increases depending on the number of bromine atoms in the BODIPY molecule [31]. CBD 2 exhibited consistent photosensitization ability with previously reported brominated BODIPY compounds [31]. Although CBD 1 generated very good singlet oxygen (ΦΔ=0.83), it was seen that CBD 1 was less efficient compared to similar iodinated BODIPY-monomer (ΦΔ=0.94) [32]. CBD 1 displayed competitive singlet oxygen production compared to BODIPY-dimer (ΦΔ=0.84) [32]. Also, both dendrimeric compounds produced singlet oxygen in good yields compared to conventional photosensitizer methylene blue (φΔ= 0. 57 in DCM) [27]. CBD 1, in particular, exhibited very high φΔ value of 0.83, making it excellent singlet oxygen photosensitizer. Secondly,

singlet-oxygen

phosphorescence

peaks

at

1270

nm

for

dendrimeric

cyclotriphosphazenes (CBD 1 and CBD 2) and MB as a standard were sensed in dichloromethane solutions. The equal absorbances (0.236) for the photosensitizers (CBD 1, CBD 2 and MB) were adjusted and then the photosensitizers were excited by using a xenonarc source at their regarding absorption maxima and detected with a near-IR detector (Fig. 5). The equal absorptions of dendrimeric cyclotriphosphazenes (CBD 1 and CBD 2) substituted

15

with heavy atoms (iodine and bromine) and MB dyads were carried out in dichloromethane whereas CBD 1 produced the most potent phosphorescence emission band at its signature wavelength (1270 nm). Since equal absorbances of the photosensitizers were studied, the area under the phosphorescence peaks can be directly correlated. It is clear that the CBD 2 was also produced singlet oxygen more powerful than MB. By using Eq. 2, the singlet oxygen quantum yields were calculated as 0.84 and 0.61 for CBD 1 and CBD 2, respectively. The results showed that the 1O2 production of CBD 1 is more efficient than that of CBD 2. The effective increase in φΔ can be explained by halogen atom effect that is responsible for the increase of the spin-orbital coupling (SOC) and the yield of intersystem crossing (ISC) [4]. The decrease of fluorescence quantum yield and lifetime, and increase of the singlet oxygen quantum yields can be explained by enhanced ISC depending on the size of the halogen atom. Also, dichloromethane solution of CBD 1 and CBD 2 without DPBF were stimulated with green LED source for 20 min and no substantial change was observed in the absorbance intensities which displays the photo-stability of the dendrimeric cyclotriphosphazenes (Fig. S16 and S17). Another important advantage of dendrimeric systems is that their working concentrations may be lower than the monomer units. The working concentration of dendrimeric dyads was determined as 0.5 μM and this value is smaller than the values of many other reported photosensitizers [31-34].

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(a)

(b)

1.2

1.2

y = -0.0249x + 0.9479 R² = 0.9923

Max.abs

1

1

Absorbance Linear Fit of absorbance

0.8 0.6 0.4 0.2

0.8

Absorbance

0 0

10

0.6

Time (sec)

20

30

0 sec dark 5 sec 10 sec 15 sec 20 sec 25 sec 30 sec

0.4

0.2

0 350

400

450

500 550 Wavelength (nm)

(c)

(d)

1.2 1

Max.abs

1.2

1

600

y = -0.0234x + 1.0612 R² = 0.9968

650

700

Absorbance Linear Fit of absorbance

0.8 0.6 0.4 0.2

0.8

0

Absorbance

0

10

20 Time (sec)

30

0.6

0 sec dark 5 sec 10 sec 15 sec 20 sec 25 sec 30 sec 35 sec

0.4

0.2

0 350

400

450

500 550 Wavelength (nm)

600

650

700

Figure 4. (a) Decrease in absorbance spectrum of DPBF in the presence of CBD 1 (0.5 µM) in dichloromethane. (b) Absorbance decrease of DPBF at 414 nm by time in dichloromethane in the presence of CBD 1. (c) Decrease in absorbance spectrum of DPBF in the presence of CBD 2 (0.5 µM) in dichloromethane. (d) Absorbance decrease of DPBF at 414 nm by time in dichloromethane in the presence of CBD 2.

17

Figure 5. Singlet oxygen phosphorescence with sensitization from CBD 1, CBD 2 and MB in dichloromethane

4. Conclusions In this study, we reported the synthesis, characterization, photophysical, and photochemical properties of the dendrimeric systems containing BODIPY units with different halogen atoms. The molecular structures of the novel BODIPY compound (3) and BODIPYcyclotriphosphazene dyads (CBD 1 and CBD 2) were characterized by NMR (31P and 1H, 13C) spectroscopies. The structure of the di-halogenated BODIPY derivatives (2 and 3) were also determined by single crystal X-ray crystallography. The dendrimeric cyclotriphosphazeneBODIPY systems displayed strong absorption bands with good molar extinction coefficients whereas the fluorescence quantum yield of the dendrimeric systems were calculated very low. The singlet oxygen quantum yields of CBD 1 and CBD 2 were determined to be 0.83 and 0.64, respectively by chemical trapping method. The results revealed that the nature of the halogen atoms had strong influence on the photo-degradation process of DPBF by the generated 1O2.

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The decrease in the fluorescence quantum yield and increase in the singlet oxygen quantum yields can be explained by enhanced ISC depending on the size of the halogen atom. The photosensitization ability of the both dendrimeric cyclotriphosphazene-BODIPY systems are stronger than the conventional photosensitizers. The present study suggested the possible use of the dendrimeric cyclotriphosphazene systems in photochemistry, photobiology and especially photocatalytic application.

Acknowledgements This work was supported by the TUBITAK project no: 114-Z- 445.

Appendix A. Supplementary material Supplementary data were given as Supporting Information. Structure determinations have been deposited with the Cambridge Crystallographic Data Centre with references CCDC 1951460 and 1951461 for compounds 2 and 3. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif

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Author Contribution Statement Dr.Elif ŞENKUYTU: Investigation, Formal analysis Dr.Elif OKUTAN: Validation, Writing - Original Draft Dr.Esra TANIRIVERDİ EÇİK: Conceptualization, Project administration, Writing - Review & Editing

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Highlights  Synthesis of cyclotriphosphazene-BODIPY dyads  Investigation of photophysical and photochemical properties  Generation of singlet oxygen

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Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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